1. Field of the Invention
This invention relates to a method for preparing 2-pyrazolin-5-ones. More particularly this invention concerns a method for preparing 2-pyrazolin-5-ones from 1,2,4-oxadiazoles. In one aspect this invention relates to a process for preparing 2-pyrazolin-5-ones that are particularly useful as color forming couplers or as intermediates in preparing color forming couplers which produce dyes in photographic elements.
2. Description Relative to the Prior Art
It is well known in the photographic art that 2-pyrazolin-5-ones represented by the structural formula ##STR1## (wherein each of R.sup.1, R.sup.2 and R.sup.3 is hydrogen, halo, alkyl, or alkoxy, and Z is an alkyl, acyl or aryl radical) are useful as color forming couplers or as intermediates in preparing color forming couplers useful for producing dyes in photographic elements. Many references describe the use of such 2-pyrazolin-5-ones in photographic elements and also describe methods for synthesizing such compounds. See, for example, U.S. Pat. Nos. 3,419,391; 3,062,653; 2,865,748; 2,600,788; and the patents referenced therein, all of which are hereby incorporated herein by reference.
It would be desirable to have a new synthetic route to 2-pyrazolin-5-ones which is more efficient and more economical than the previously known methods. The present invention unexpectedly provides such a synthetic route. It involves the rearrangement of certain 1,2,4-oxadiazole to their corresponding 2-pyrazolin-5-ones.
In general regard to oxadiazole rearrangements, U.S. Pat. No. 3,927,025, to Korbonits et al, and Ruccia et al, A New Rearrangement in the 1,2,4-Oxadiazole Series, in J. Chem. Soc. Chem. Comm., p. 866 (1970) are relevant references. These references describe rearrangements of 1,2,4-oxadiazoles, but the rearrangements described are carried out in aqueous solutions and yield pyrazolines or triazolinones, which are, chemically, compounds significantly different from 2-pyrazolin-5-ones. These references do not describe or suggest any new synthetic routes to 2-pyrazolin-5-ones.